1. Field of the Invention
The present invention relates to an image pickup apparatus having a high resolution and a high signal-to-noise (S/N) ratio using a confocal optical system.
Further, the present invention relates to a photomask defect inspection system having such an image pickup apparatus.
2. Description of the Related Art
In the process of production of a semiconductor device, a large number of photomasks formed with various different patterns are used. A desired device is manufactured by repeating the steps of pattern transfer and etching on a semi-conductor wafer. When there is a defect in the photomask serving as the master for pattern transfer due to foreign matter etc., an accurate pattern cannot be projected on the semi-conductor wafer and defects end up occurring there. Therefore, there has been a strong demand for improvement of a defect inspection system for detecting a defect of a photomask.
In the past, in a photomask defect inspection system, the photomask to be inspected has been scanned at a high speed by light beam, the light transmitted through or the light reflected from the photomask has been received by a one-dimensional image sensor, and output signals from the image sensor are compared with data stored in a database or compared output signals with each other to detect the presence of foreign matters and the detects of a light blocking pattern.
In an another defect inspection system, the light transmitted through or the light reflected from the photomask is received by a two-dimensional CCD camera, and output signals from the light receiving elements of the CCD camera have been compared with data stored in a database or compared with each other to detect a defect.
Along with the higher integration and higher density of LSIs, the patterns of photomasks have also become finer. There has therefore been a strong demand to further raise the resolution of a photomask defect inspection system and obtain a defect detection signal of a high S/N ratio. The method of receiving the light from the above photomask by a one-dimensional image sensor has the advantage of giving a relatively high resolution since the confocality is maintained. The charge stored in a light receiving element of the image sensor, however, is proportional to the time of irradiation of the illumination light, that is, the time of accumulation of the charge. In the method of high speed scanning using a light beam, the time during which the light from the photomask strikes the image sensor is short, so the amount of charge stored in a light receiving element becomes smaller and there are limits in regard to the S/N ratio of the defect detection signal.
On the other hand, in the method of receiving light from a photomask by a two-dimensional CCD camera, the illumination time can be made relatively longer, so a good characteristic is obtained with respect to the S/N ratio. When picking up a reflected image or transmitted image of the photomask by a two-dimensional CCD camera, flare, glare, and other stray light ends up striking the light receiving elements, there is a limit to the resolution, and there is a limit to the inspection for defects of a fine pattern.
Further, to optically inspect for defects of a fine pattern, it is desirable to use light of a short wavelength, that is, ultraviolet light, as the illumination light. With ultraviolet light, however, since the absorption by the optical elements is large and the sensitivity of the photodiodes is low, there is the disadvantage of difficulty of obtaining a sufficient detection sensitivity with a defect inspection system of the related art. Further, among the types of defects are defects due to the deposition of foreign matter and pattern defects derived from the failure of accurate formation of a chrome light blocking pattern. If these types of defects can be discerned, the applications for defect inspection systems can be further increased.
An object of the present invention is to provide an image pickup apparatus and photomask defect inspection system able to pick up the image of a sample at a higher resolution than a defect inspection system of the related art and obtain an output signal of a higher S/N ratio.
Another object of the present invention is to provide a defect inspection system able to simultaneously inspect for a defect by transmitted light and inspect for a defect by reflected light and able to discern the properties and details of the detected defect.
According to a first aspect of the present invention, there is provided an image pickup apparatus provided with a sample stage for moving a sample whose image is to be picked up in a first direction; an illumination light source for projecting illumination light on the sample; a first spatial filter arranged between the illumination light source and sample and having a plurality of slits formed at a predetermined pitch along the first direction and extending in a second direction perpendicular to the first direction; an image sensor for receiving reflected light or transmitted light from a sample, having a plurality of light receiving elements arranged in a two-dimensional array along the first and second directions, successively transferring the charges stored in one line of light receiving elements arranged in the second direction for each line at a predetermined transfer speed, and successively outputting the charges stored in the light receiving elements; a second spatial filter arranged between the sample stage and the second image sensor and having a plurality of slits formed at a predetermined pitch along the first direction and extending in the second direction; an objective lens arranged between the sample stage and second spatial filter and forming an image of transmitted light or reflected light from the sample on the image sensor via the slits of the second spatial filter; and a drive control circuit for controlling the charge transfer speed of the image sensors and speed of movement of the sample stage; the first and second spatial filters being arranged so that light emitted from the slits of the first spatial filter strikes the sample and the image sensor through the slits of the second spatial filter; the speed of movement of the sample stage and the charge transfer speed of the image sensor being set so that the time interval during which the sample moves from the position which the illumination light passing through an ixe2x88x92th slit of the first spatial filter strikes to the position which the illumination light passing through the adjoining i+1st slit strikes and the time interval during which the charges stored in the line of light receiving elements, of the image sensor, which the light passing through the ixe2x88x92th slit of the second spatial filter strikes are transferred to the line of light receiving elements which the illumination light passing through the adjoining i+1st slit strikes become equal to each other.
In the present invention, since the illumination light is projected as lines of illumination light through a spatial filter having a plurality of slits extending in a direction perpendicular to a direction of movement of the sample and the transmitted light or reflected light from the sample is received at an image sensor through a spatial filter having a plurality of slits extending in a direction perpendicular to the direction of movement of the sample in the same way, a line confocal optical system is formed and as a result it is possible to pick up an image greatly reduced in flare, glare, and other stray light and having a high resolution.
Further, since the speed of movement of the sample and the line transfer speed of the image sensor are made to correspond to each other, that is, since the speed of movement of the sample stage and charge transfer speed of the image sensor are set so that the time during which the sample moves from the position which the illumination light passing through an ixe2x88x92th slit of the first spatial filter strikes to the position which the illumination light passing through the adjoining i+1st slit strikes and the time interval during which the charges stored in the line of light receiving elements, of the image sensor, which the light passing through the ixe2x88x92th slit of the second spatial filter strikes are transferred to the line of light receiving elements which the illumination light passing through the adjoining i+1st slit strikes become equal to each other, the same location of the sample is illuminated a plurality of times and the charges generated due to the plurality of instances of illumination light build up, so the noise can be greatly reduced and the S/N ratio can be greatly improved. As a result, it is possible to simultaneously improve both the resolution and S/N ratio and possible to realize a defect inspection system of a much higher accuracy by using a pickup apparatus having such a high resolution and high S/N ratio as an image pickup optical system of a photomask defect inspection system.
Preferably, the image pickup apparatus is further provided with a position detection device for detecting a position of the sample stage in the first direction, the drive control circuit adjusting a charge transfer speed of the image sensor based on a stage position signal from the position detection device. By adopting such a configuration, the correspondence between the image sensor and stage is maintained even if the speed of movement of the stage shifts from a reference value, a clear image can be picked up at all times, and therefore the accuracy of the detection of defects can be maintained.
According to a second aspect of the present invention, there is provided an image pickup apparatus provided with a sample stage for moving a sample whose image is to be picked up in a first direction; a first light source for projecting illumination light for picking up a transmitted image of a sample; a second light source for projecting illumination light for picking up a reflected image of a sample; a first spatial filter arranged between the first light source and the sample stage and having a plurality of slits formed at a predetermined pitch along the first direction and extending in a second direction perpendicular to the first direction; a second spatial filter arranged between the second light source and the sample stage and having a plurality of slits formed at a predetermined pitch along the first direction and extending in the second direction perpendicular to the first direction; a first image sensor for receiving light generated from the first light source and passing through the sample, having a plurality of light receiving elements arranged in a two-dimensional array along the first and second directions, successively transferring the charges stored in one line of light receiving elements arranged in the second direction for each line at a predetermined transfer speed, and successively outputting the charges stored in the light receiving elements; a second image sensor for receiving light generated from the second light source and reflected at the sample, having a plurality of light receiving elements arranged in a two-dimensional array along the first and second directions, successively transferring the charges stored in one line of light receiving elements arranged in the second direction for each line at a predetermined transfer speed, and successively outputting the charges stored in the light receiving elements; a third spatial filter arranged between the sample stage and first image sensor and having a plurality of slits formed at a predetermined pitch along the first direction and extending in the second direction; a fourth spatial filter arranged between the sample stage and second image sensor and having a plurality of slits formed at a predetermined pitch along the first direction and extending in the second direction; an object lens arranged between the sample stage and third and fourth spatial filters and forming an image of transmitted light and reflected light from the sample on the first and second image sensors via the slits of the third and fourth spatial filters; and a drive control circuit for controlling the charge transfer speed of the first and second image sensors and speed of movement of the sample stage; the first spatial filter and third spatial filter being arranged so that illumination light emitted from the slits of the first spatial filter strike the sample, object lens, and image sensor through the slits of the third spatial filter; the second spatial filter and fourth spatial filter being arranged so that illumination light emitted from the slits of the second spatial filter strike the sample, object lens, and image sensor through the slits of the fourth spatial filter; the speed of movement of the sample stage and the charge transfer speed of the first and second image sensors being set so that the time during which the sample moves from the position which the illumination light passing through an ixe2x88x92th slits of the first and second spatial filters strikes to the position which the illumination light passing through the adjoining ixe2x88x921st slits strikes and the time during which the charges stored in the line of light receiving elements, of the image sensors, which the light passing through the ixe2x88x92th slits of the third and fourth spatial filters strikes are transferred to the line of light receiving elements which the illumination light passing through the adjoining i+1st slits strikes become equal to each other.
This image pickup apparatus can simultaneously pick up a reflected image and transmitted image of the sample. In this image pickup apparatus, by making the wavelength of the transmitted illumination light and the wavelength of the reflected illumination light different each other and arranging a dichroic mirror between the sample and image sensors 1, it is possible to easily separate the transmitted light and reflected light from the sample each other. When this image pickup apparatus is used for inspection of defects in a photomask, output signals of the first and second image sensors are supplied to a defect detection circuit, the output signals of the two image sensors are added, and the resultant output signal is compared with a threshold value by a comparison circuit to detect the presence of foreign matter.
Preferably, the first and second light sources generate illumination light of equal wavelengths and the third and fourth spatial filters are arranged so that the transmitted light from the sample strikes the light blocking portions between slits of the fourth spatial filter and the reflected light from the sample strikes the light blocking portions between the slits of the third spatial filter.
Preferably, the wavelength of the illumination light generated from the first light source and the wavelength of the illumination light generated from the second light source are made different from each other and a separation element for separating the transmitted light and reflected fight from the sample is arranged between the sample and the third and fourth spatial filters.
According to a third aspect of the present invention, there is provided an image pickup apparatus provided with a sample stage for moving a sample whose image is to be picked up in a first direction; a light source for generating a light beam; a diffraction grating for generating n number of sub beams at predetermined intervals along a direction corresponding to the first direction from the light beam; a beam deflection device for cyclically deflecting the sub beams by a predetermined frequency in a second direction perpendicular to the first direction; an image sensor for receiving reflected light or transmitted light from a sample, having a plurality of light receiving elements arranged in a two-dimensional array along the first and second directions, successively transferring the charges stored in one line of light receiving elements arranged in the second direction for each line at a predetermined transfer speed, and successively outputting the charges stored in the light receiving elements; a spatial filter arranged between the sample stage and image sensor and having a plurality of slits formed at a predetermined pitch along the first direction and extending in the second direction; an object lens arranged forming an image of transmitted light or reflected light from the sample on the image sensor via the slits of the second spatial filter; and a drive control circuit for controlling the drive of the image sensor and beam deflection apparatus; the reflected light or transmitted light due to the plurality of sub beams scanning the surface of the sample striking the image sensor through the slits of the spatial filter; the speed of movement of the sample stage and the charge transfer speed of the image sensor being set so that the time interval during which the sample moves from the position which an ixe2x88x92th sub beam strikes to the position which the adjoining i+1st sub beam strikes and the time interval during which the charges stored in the line of light receiving elements, of the image sensor, which the light passing through the ixe2x88x92th slit of the spatial filter strikes are transferred to the line of light receiving elements which the illumination light passing through the adjoining i+1st slit strikes become equal to each other.
In this way, since a confocal optical system is formed by using multi beam illumination and the sample is illuminated a plurality of times, a sample having a high resolution and high S/N ratio can be picked up. Therefore, by using an image pickup apparatus as an image pickup optical system of a photomask defect inspection system, it is possible to detect defects of a photomask by a higher solution and higher S/N ratio.
According to a fourth aspect of the present invention, there is provided an image pickup apparatus provided with a sample stage for moving a sample whose image is to be picked up in a first direction; a light source for generating illumination light for picking up a transmitted image; a first spatial filter arranged between the illumination light source and sample and having a plurality of slits formed at a predetermined pitch along the first direction and extending in a second direction perpendicular to the first direction; a first image sensor for receiving transmitted light from a sample, having a plurality of light receiving elements arranged in a two-dimensional array along the first and second directions, successively transferring the charges stored in one line of light receiving elements arranged in the second direction for each line at a predetermined transfer speed, and successively outputting the charges stored in the light receiving elements; a second spatial filter arranged between the sample and first image sensor and having a plurality of slits formed at a predetermined pitch along the first direction and extending in the second direction; a light source for generating a light beam for picking up a reflected image; a diffraction grating for generating n number of sub beams from the light beam; a beam deflection device for deflecting the sub beams by a predetermined frequency in a second direction perpendicular to the first direction; a second image sensor for receiving reflected light from a sample, having a plurality of light receiving elements arranged in a two-dimensional array along the first and second directions, successively transferring the charges stored in one line of light receiving elements arranged in the second direction for each line at a predetermined transfer speed, and successively outputting the charges stored in the light receiving elements; a third spatial filter arranged between the sample stage and second image sensor and having a plurality of slits formed at a predetermined pitch along the first direction and extending in the second direction; an objective lens arranged forming an image of transmitted light and reflected light from the sample on the image sensors via the slits of the second and third spatial filters; and a drive control circuit for controlling the drive of the first and second image sensors and beam deflection apparatus; the first and second spatial filters being arranged so that light emitted from the slits of the first spatial filter strike the sample, the object lens, and the first image sensor through the slits of the second spatial filter; the third spatial filter being arranged so that reflected light from the sample strikes the objective lens and the second image sensor through the slits of the third spatial filter; the speed of movement of the sample stage and the charge transfer speed of the image sensors being set so that the time during which the sample moves from the position which the ixe2x88x92th sub beam strikes to the position which the adjoining i+1st sub beam strikes and the time during which the charges stored in the line of light receiving elements, of the image sensors, which the light passing through the ixe2x88x92th slit of the spatial filter strikes is transferred to the line of light receiving elements which the illumination light passing through the adjoining i+1st slit strikes become equal to each other. In this embodiment, the reflected image pickup optical system and transmitted image pickup optical system are formed by confocal optical systems, so a high resolution high S/N ratio image can be picked up.
According to a fifth aspect of the present invention, there is provided an image pickup apparatus provided with a sample stage for moving a sample whose image is to be picked up in a first direction; an illumination light source for projecting illumination light for picking up a transmitted image; a first spatial filter arranged between the illumination light source and sample and having a plurality of slits formed at a predetermined pitch along the first direction and extending in a second direction perpendicular to the first direction; a first image sensor for receiving transmitted light from a sample, having a plurality of light receiving elements arranged in a two-dimensional array along the first and second directions, successively transferring the charges stored in one line of light receiving elements arranged in the second direction for each line at a predetermined transfer speed, and successively outputting the charges stored in the light receiving elements; a second spatial filter arranged between the sample and first image sensor and having a plurality of slits formed at a predetermined pitch along the first direction and extending in the second direction; a light source for generating a light beam for picking up a reflected image; a beam deflection apparatus for deflecting the light beam by a predetermined frequency in a second direction perpendicular to the first direction; a linear image sensor for receiving reflected light from the sample, having a plurality of light receiving elements arranged in a line along the second direction, and reading out the charges stored in the light receiving elements in synchronization with the first image sensor; an objective lens arranged for forming an image of transmitted light and reflected light from the sample on the first image sensor and linear image sensor; and a drive control circuit for controlling the drive of the first and linear image sensors and the beam deflection apparatus; the first and second spatial filters being arranged so that light emitted from the slits of the first spatial filter strike the sample, the object lens, and the image sensor through the slits of the second spatial filter; the speed of movement of the sample stage and the charge transfer speed of the image sensors being set so that the time during which the sample moves from the position which the ixe2x88x92th sub beam strikes to the position which the adjoining i+1st sub beam strikes and the time during which the charges stored in the line of light receiving elements, of the image sensors, which the light passing through the ixe2x88x92th slit of the spatial filters strikes are transferred to the line of light receiving elements which the illumination light passing through the adjoining i+1st slit strikes become equal to each other. By this defect inspection system, it is possible to detect a defect of a photomask by a die-to-die or chip-to-chip relationship. It is therefore possible to defect fine defects at a high speed and with a high accuracy.
Preferably, the beam deflection apparatus is comprised by an acoustic optical element and the beam deflection frequency of the acoustic optical element is set to a whole multiple (including an equal value) of the charge transfer speed of the image sensors.
More preferably, each light receiving element of the image sensors has a charge storing ability able to store a charge generated in accordance with light striking it.
In accordance with a sixth aspect of the present invention, there is provided a photomask defect inspection system provided with a stage for moving a photomask to be inspected for defects in a first direction; an illumination light source for projecting illumination light on the photomask; a first spatial filter arranged between the illumination light source and stage and having a plurality of slits formed at a predetermined pitch along the first direction and extending in a second direction perpendicular to the first direction; a first image sensor for receiving transmitted light from the photomask, having a plurality of light receiving elements arranged in a two-dimensional array along the first and second directions, successively transferring the charges stored in one line of light receiving elements arranged in the second direction for each line at a predetermined transfer speed, and successively outputting the charges stored in the light receiving elements; a second spatial filter arranged between the stage and first image sensor and having a plurality of slits formed at a predetermined pitch along the first direction and extending in the second direction; a light source for generating a light beam for picking up a reflected image; a diffraction grating for generating n number of sub beams from the light beam at equal intervals along a direction corresponding to the first direction; a beam deflection apparatus for cyclically deflecting the sub beams by a predetermined frequency in a second direction perpendicular to the first direction; a second image sensor for receiving reflected light from the photomask, having a plurality of light receiving elements arranged in a two-dimensional array along the first and second directions, successively transferring the charges stored in one line of light receiving elements arranged in the second direction for each line at a predetermined transfer speed, and successively outputting the charges stored in the light receiving elements; a third spatial filter arranged between the stage and second image sensor and having a plurality of slits extending in the second direction; an object lens for forming an image of transmitted light and reflected light from the sample on the first and second image sensors via the slits of the second and third spatial filters; a drive control circuit for controlling the drive of the first and second image sensors and the beam deflection apparatus; and a defect detection circuit for detecting defects of the photomask based on the output signals of the first and second image sensors; the first and second spatial filters being arranged so that the light emitted from the slits of the first spatial filter enter the slits of the second spatial filter; the third spatial filter being arranged so that the reflected light of the plurality of sub beams scanning the surface of the photomask enter the slits of the third spatial filter; the charge transfer speed of the first and second image sensors and the speed of movement of the stage being set so that the time interval during which the sample moves from the position which the ixe2x88x92th sub beam strikes to the position which the adjoining i+1st sub beam strikes and the time interval during which the charges stored in the line of light receiving elements, of the image sensor, which the light passing through the ixe2x88x92th slit of the spatial filter strikes are transferred to the line-of light receiving elements which the illumination light passing through the adjoining i+1st slit strikes become equal to each other.
According to a seventh aspect of the present invention, there is provided a photomask defect inspection system provided with a stage for moving a photomask to be inspected for defects in a first direction, first and second image pickup apparatuses for picking up an image of the photomask, and a defect detection circuit for detecting defects existing in a photomask based on output signals of the first and second image pickup apparatuses; each of the first and second image pickup apparatuses being provided with an illumination light source for projecting illumination light on the photomask; a first spatial filter arranged between the illumination light source and stage and having a plurality of slits formed at a predetermined pitch along the first direction and extending in a second direction perpendicular to the first direction; a first image sensor for receiving transmitted light from the photomask, having a plurality of light receiving elements arranged in a two-dimensional array along the first and second directions, successively transferring the charges stored in one line of light receiving elements arranged in the second direction for each line at a predetermined transfer speed, and successively outputting the charges stored in the light receiving elements; a second spatial filter arranged between the stage and first image sensor and having a plurality of slits formed at a predetermined pitch along the first direction and extending in the second direction; a light source for generating a light beam for picking up a reflected image; a diffraction grating for generating n number of sub beams from the light beam at equal intervals along a direction corresponding to the first direction; a beam deflection apparatus for cyclically deflecting the sub beams by a predetermined frequency in a second direction perpendicular to the first direction; a second image sensor for receiving reflected light from the photomask, having a plurality of light receiving elements arranged in a two-dimensional array along the first and second directions, successively transferring the charges stored in one line of light receiving elements arranged in the second direction for each line at a predetermined transfer speed, and successively outputting the charges stored in the light receiving elements; a third spatial filter arranged between the stage and second image sensor and having a plurality of slits extending in the second direction; an object lens for forming an image of transmitted light and reflected light from the sample on the first and second image sensors via the slits of the second and third spatial filters; and a drive control circuit for controlling the drive of the first and second image sensors and the beam deflection apparatus; the first and second spatial filters being arranged so that the light emitted from the slits of the first spatial filter enter the slits of the second spatial filter; the third spatial filter being arranged so that the reflected light of the plurality of sub beams scanning the surface of the photomask enter the slits of the third spatial filter; the charge transfer speed of the first and second image sensors and the speed of movement of the stage being set so that the time interval during which the sample moves from the position which the ixe2x88x92th sub beam strikes to the position which the adjoining i+1st sub beam strikes and the time interval during which the charges stored in the line of light receiving elements, of each image sensor, which the light passing through the ixe2x88x92th slit of the spatial filter strikes are transferred to the line of light receiving elements which the illumination light passing through the adjoining i+1st slit strikes become equal to each other.
Preferably, the defect detection circuit is provided with a comparison circuit for comparing output signals of the first and second image sensors of the first image pickup apparatus and output signals of the first and second image sensors of the second image pickup apparatus and defects are detected based on the results of the comparison circuit.